Effect of Different Solvents on Morphology and Gas-Sensitive Properties of Grinding-Assisted Liquid-Phase-Exfoliated MoS2 Nanosheets

Grinding-assisted liquid-phase exfoliation is a widely used method for the preparation of two-dimensional nanomaterials. In this study, N-methylpyrrolidone and acetonitrile, two common grinding solvents, were used during the liquid-phase exfoliation for the preparation of MoS2 nanosheets. The morphology and structure of MoS2 nanosheets were analyzed via scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The effects of grinding solvents on the gas-sensing performance of the MoS2 nanosheets were investigated for the first time. The results show that the sensitivities of MoS2 nanosheet exfoliation with N-methylpyrrolidone were 2.4-, 1.4-, 1.9-, and 2.7-fold higher than exfoliation with acetonitrile in the presence of formaldehyde, acetone, and ethanol and 98% relative humidity, respectively. MoS2 nanosheet exfoliation with N-methylpyrrolidone also has fast response and recovery characteristics to 50–1000 ppm of CH2O. Accordingly, although N-methylpyrrolidone cannot be removed completely from the surface of MoS2, it has good gas sensitivity compared with other samples. Therefore, N-methylpyrrolidone is preferred for the preparation of gas-sensitive MoS2 nanosheets in grinding-assisted liquid-phase exfoliation. The results provide an experimental basis for the preparation of two-dimensional materials and their application in gas sensors.


Introduction
Given the special structure and potential applications, two-dimensional (2D) materials such as graphene, boron nitride, and molybdenum disulfide (MoS 2 ) draw plenty of concerns. Among them, MoS 2 as the frontrunner in transition metal dichalcogenides (TMDCs) materials has gained the most attention [1][2][3][4] and is used in a wide variety of applications [5][6][7][8][9][10][11] due to its unique properties [12][13][14]. MoS 2 is at the forefront in the race of an ideal gas-sensing material because of its large surface-to-volume ratio, enormous number of active sites, and favorable adsorption sites [15,16]. MoS 2 manifests two possible crystal phases, including trigonal and hexagonal structures, with metallic and semiconducting properties, respectively [17]. The presence of weak Van der Waals force facilitates the isolation of layers from bulk MoS 2 . The indirect bandgap of 1.2 eV in bulk MoS 2 is converted to a direct bandgap of 1.8 eV for monolayer MoS 2 [3,14,18]. The absence of dangling bonds provides stability to pristine MoS 2 flakes in liquid and gaseous media in the presence of oxygen, thereby facilitating its gas-sensing application [19,20]. Therefore, a reliable and low-cost technique is needed to produce 2D-MoS 2 for gas-sensing applications. Currently, several methods including vapor deposition [21], mechanical exfoliation [22], lithium-ion intercalation [23], liquid-phase exfoliation [24,25], and RF sputtering [26] have

Preparation of Materials
MoS 2 , with a purity of 99% and particle size less than 2 µm, was purchased from Sigma-Aldrich. ACN, NMP, and absolute ethanol (C 2 H 6 O) were purchased from Tianjin Zhiyuan Chemical Reagent Co. Ltd. as analytically pure reagents. The preparation of MoS 2 nanosheets via grinding-assisted liquid-phase exfoliation is described as follows: MoS 2 powder (100 mg) was manually ground in a mortar for 2 h, and 0.5 mL of the chosen solvent was added during the grinding. The sample was then dried in a vacuum oven at 60 • C for 12 h. The dried sample was dispersed in 40 mL of 45 vol% absolute ethanol and sonicated for 1 h at 120 W with stirring. The dispersion was centrifuged for another 20 min (1500 r / min) to obtain the MoS 2 nanosheets, and the supernatant was dried in air for further use. For convenience, the MoS 2 nanosheets obtained by grinding with ACN were designated as S1, and those ground with NMP were called S2.

Characterizations
The morphology of MoS 2 nanosheets was observed with a field emission scanning electron microscope (SEM, JSM-7610F Plus). The crystal structure of MoS 2 nanosheets was characterized by X-ray diffraction (XRD, Bruker D8 Advance, with Cu-Kα radiation). Raman spectroscopy (Renishaw inVia, Gloucester, Britain) was used to characterize the defects and functional groups of samples. The I-t and I-V curves of the sensing chip were measured by Keithley 2636B at room temperature.

Device Fabrication and Testing
The MoS 2 nanosheets were dispersed in absolute ethanol at 10 mg/mL. Dispersions (2 µL) were uniformly coated to fabricate a MoS 2-based sensing chip with Ag-Pd fork-finger electrodes. The minimum width and spacing of electrodes was 0.2 mm. The interdigital electrode was dried at 25 • C and aged for 24 h at a voltage of 4 V to obtain a sensing chip with good stability. The target vapor was produced by thermal evaporation, according to our previous work [35], and a calculated amount of target liquid was dropped onto a hot plate in a 1 L container to generate target vapor in the container. Next, 98% relative humidity was obtained by saturating salt solution (potassium sulphate-K 2 SO 4 ). Then, by transferring the sensing chip from the air to the target gas at room temperature, the Keithley 2636B recorded the change of the current signal of the sensing chip ( Figure S1).
The response was defined using the formula I G −I R I R × 100%, where I R and I G are the currents of the sensor in the reference gas and target gas, respectively. The response time and recovery time were defined as the response values of 90% and 10% of the current of the sensor in contact with the target gas, respectively.

Results and Discussion
The XRD patterns of the two types of MoS 2 prepared by different grinding solvents are shown in Figure 1 Figure S2) of the peaks appeared after liquid-phase exfoliation, indicating that the MoS 2 nanosheets were able to be exfoliated, and thus, the size of MoS 2 decreases [36][37][38][39]. man spectroscopy (Renishaw inVia, Gloucester, Britain) was used to characterize the defects and functional groups of samples. The I-t and I-V curves of the sensing chip were measured by Keithley 2636B at room temperature.

Device Fabrication and Testing
The MoS2 nanosheets were dispersed in absolute ethanol at 10 mg/mL. Dispersions (2 μL) were uniformly coated to fabricate a MoS2-based sensing chip with Ag-Pd forkfinger electrodes. The minimum width and spacing of electrodes was 0.2 mm. The interdigital electrode was dried at 25°C and aged for 24 h at a voltage of 4 V to obtain a sensing chip with good stability. The target vapor was produced by thermal evaporation, according to our previous work [35], and a calculated amount of target liquid was dropped onto a hot plate in a 1 L container to generate target vapor in the container. Next, 98% relative humidity was obtained by saturating salt solution (potassium sulphate-K₂SO₄). Then, by transferring the sensing chip from the air to the target gas at room temperature, the Keithley 2636B recorded the change of the current signal of the sensing chip ( Figure S1). The response was defined using the formula ( − ) × 100%, where IR and IG are the currents of the sensor in the reference gas and target gas, respectively. The response time and recovery time were defined as the response values of 90% and 10% of the current of the sensor in contact with the target gas, respectively.

Results and Discussion
The XRD patterns of the two types of MoS2 prepared by different grinding solvents are shown in Figure 1 Figure S2) of the peaks appeared after liquid-phase exfoliation, indicating that the MoS2 nanosheets were able to be exfoliated, and thus, the size of MoS2 decreases [36][37][38][39].   Raman spectroscopy is effective in distinguishing bulk from exfoliated 2D materials. Figure 2 shows the Raman spectra of bulk MoS 2 : S1 and S2. The two Raman peaks correspond to the high-energy A 1g mode and lower-energy E 1 2g mode. As shown in Figure 2a, all the samples displayed the E 1 2g and A 1g peaks of MoS 2 . Comparing with peaks of bulk MoS 2 , a red shift of E 1 2g peak and a blue shift of the A 1g peak were observed for both S1 and S2, respectively. These shifts are associated with nanosheets obtained with NMP and ACN [40,41]. Figure 2b presents two very broad and intense Raman peaks (1360 and 1580-cm −1 ) of S2, which may be assigned to NMP [31,36] that was not completely removed from the surface of MoS 2 nanosheets although it was heated and reduced at 60 • C for several hours. In contrast, S1 showed no broad peaks, indicating that ACN was almost removed. Raman spectroscopy is effective in distinguishing bulk from exfoliated 2D materials. Figure 2 shows the Raman spectra of bulk MoS2: S1 and S2. The two Raman peaks correspond to the high-energy 1 mode and lower-energy 2g 1 mode. As shown in Figure 2a, all the samples displayed the 2g 1 and 1 peaks of MoS2. Comparing with peaks of bulk MoS2, a red shift of 2g 1 peak and a blue shift of the 1 peak were observed for both S1 and S2, respectively. These shifts are associated with nanosheets obtained with NMP and ACN [40,41]. Figure 2b presents two very broad and intense Raman peaks (1360 and 1580cm −1 ) of S2, which may be assigned to NMP [31,36] that was not completely removed from the surface of MoS2 nanosheets although it was heated and reduced at 60° C for several hours. In contrast, S1 showed no broad peaks, indicating that ACN was almost removed. We next investigated the effect of grinding solvents on the morphology of MoS2 nanosheets. The SEM image shown in Figure 3a,b reveals the morphology of the starting MoS2 powder as a thick layer with dimensions ranging from about 1 to 6.4 μm. The SEM images presented in Figure 3c,d clearly indicate that the lateral sizes and thicknesses of layered MoS2 were reduced by combined grinding and sonication. The MoS2 nanosheets were obtained by grinding with ACN (S1), as shown in Figure 3c,d, and the nanosheets were uniform in size and well-dispersed, with the majority measuring between 0.1 and 0.5 μm. As shown in Figure 3e,f, exfoliation with NMP (S2) also produced nanosheets with good dispersion with lateral dimensions of 0.4-1.6 μm. The MoS2 nanosheets obtained by grinding with ACN were smaller than NMP-ground MoS2 nanosheets, which is consistent with the results reported in the literature [34] and the results of XRD patterns (Figures 1  and S2). We next investigated the effect of grinding solvents on the morphology of MoS 2 nanosheets. The SEM image shown in Figure 3a,b reveals the morphology of the starting MoS 2 powder as a thick layer with dimensions ranging from about 1 to 6.4 µm. The SEM images presented in Figure 3c,d clearly indicate that the lateral sizes and thicknesses of layered MoS 2 were reduced by combined grinding and sonication. The MoS 2 nanosheets were obtained by grinding with ACN (S1), as shown in Figure 3c,d, and the nanosheets were uniform in size and well-dispersed, with the majority measuring between 0.1 and 0.5 µm. As shown in Figure 3e,f, exfoliation with NMP (S2) also produced nanosheets with good dispersion with lateral dimensions of 0.4-1.6 µm. The MoS 2 nanosheets obtained by grinding with ACN were smaller than NMP-ground MoS 2 nanosheets, which is consistent with the results reported in the literature [34] and the results of XRD patterns (Figures 1 and S2).
The gas-sensitive properties of MoS 2 nanosheets loaded on ceramic substrates were tested at room temperature. The results shown in Figure 4a,c indicate gas-sensitive properties and response time (Figure 4b,d) of S1 and S2 at 98% relative humidity (RH) and 1000 ppm of formaldehyde (CH 2 O), acetone (C 3 H 6 O), and ethanol (C 2 H 6 O). The MoS 2 layers exfoliated with both the grinding solvents showed good stability in three continuous responserecovery cycles at room temperature. Both of them completed a response-recovery cycle in 40 s and returned completely each time with almost no drift. Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 12 The gas-sensitive properties of MoS2 nanosheets loaded on ceramic substrates were tested at room temperature. The results shown in Figure 4a,c indicate gas-sensitive properties and response time (Figure 4b,d) of S1 and S2 at 98% relative humidity (RH) and 1000 ppm of formaldehyde (CH2O), acetone (C3H6O), and ethanol (C2H6O). The MoS2 layers exfoliated with both the grinding solvents showed good stability in three continuous response-recovery cycles at room temperature. Both of them completed a response-recovery cycle in 40 s and returned completely each time with almost no drift.  Figure 5 shows the average response, response time, and recovery time of S1 and S2 for the target analyte. As can be seen from Figure 5a, the sensitivities of MoS 2 nanosheets exfoliation with NMP (S2) were 2.4, 1.4, 1.9, and 2.7 times higher than exfoliation with ACN (S1) to CH 2 O, C 3 H 6 O, C 2 H 6 O, and 98%RH, respectively. These results prove that the MoS 2 nanosheets obtained by grinding with NMP have higher gas-responsive properties than the MoS 2 nanosheets with ACN although NMP was not removed completely. At the same time, it can be seen from Figure 5b,c that both samples have faster response time to the four analytes, which did not exceed 35 s, and the recovery time did not exceed 4 s. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 12 Figure 4. Sensing curves in the presence of different target gases of S1 and S2. (a) and (c) Gas-sensitive properties of S1 and S2 at 98% RH and 1000 ppm of CH2O, C3H6O, and C2H6O, respectively. (b) and (d) Response time of S1 and S2 at 98% RH and 1000 ppm of CH2O, C3H6O, and C2H6O, respectively. Figure 5 shows the average response, response time, and recovery time of S1 and S2 for the target analyte. As can be seen from Figure 5a, the sensitivities of MoS2 nanosheets exfoliation with NMP (S2) were 2.4, 1.4, 1.9, and 2.7 times higher than exfoliation with ACN (S1) to CH2O, C3H6O, C2H6O, and 98%RH, respectively. These results prove that the MoS2 nanosheets obtained by grinding with NMP have higher gas-responsive properties than the MoS2 nanosheets with ACN although NMP was not removed completely. At the same time, it can be seen from Figure 5b,c that both samples have faster response time to the four analytes, which did not exceed 35 s, and the recovery time did not exceed 4 s.   . Sensing curves in the presence of different target gases of S1 and S2. (a) and (c) Gas-sensitive properties of S1 and S2 at 98% RH and 1000 ppm of CH2O, C3H6O, and C2H6O, respectively. (b) and (d) Response time of S1 and S2 at 98% RH and 1000 ppm of CH2O, C3H6O, and C2H6O, respectively. Figure 5 shows the average response, response time, and recovery time of S1 and S2 for the target analyte. As can be seen from Figure 5a, the sensitivities of MoS2 nanosheets exfoliation with NMP (S2) were 2.4, 1.4, 1.9, and 2.7 times higher than exfoliation with ACN (S1) to CH2O, C3H6O, C2H6O, and 98%RH, respectively. These results prove that the MoS2 nanosheets obtained by grinding with NMP have higher gas-responsive properties than the MoS2 nanosheets with ACN although NMP was not removed completely. At the same time, it can be seen from Figure 5b,c that both samples have faster response time to the four analytes, which did not exceed 35 s, and the recovery time did not exceed 4 s.  In order to further evaluate the real-time monitoring capability of MoS 2 nanosheets obtained by grinding with NMP (S1), the responses of the S2-based sensor under different concentrations (50-2000 ppm) of CH 2 O vapor were evaluated (Figure 6a). The response of S2 increased with the increase of CH 2 O concentration. Figure 5b shows a linear response to changing CH 2 O concentration, and the correlation coefficient R2 was 0.99, which facilitated gas-sensing application. Figure 6a S2 increased with the increase of CH2O concentration. Figure 5b shows a linear response to changing CH2O concentration, and the correlation coefficient R2 was 0.99, which facilitated gas-sensing application. Figure 6a,b show that the response time and recovery time of S2 were only 18 s and 0.5 s to 50 ppm CH2O, respectively, and only 11 s and 0.6 s to 100 ppm CH2O. In order to comprehensively evaluate the gas-sensing performance of MoS2 nanosheets obtained by grinding with NMP, the performances of the MoS2 nanosheetbased sensors were compared ( Table 1). As shown in Table 1, the response time and recovery time of MoS2 nanosheets obtained by grinding with NMP for 50 ppm CH2O were 18 s and 0.51 s, respectively, which were close to the shortest response time (11 s) and recovery time (8 s) shown by ZnS and In2O3/MoS2 [42,43]. Nevertheless, compared with the operating temperature (295 °C) of ZnS, the operating temperature of MoS2 nanosheets was at room temperature (25 °C). Therefore, the MoS2 nanosheets exhibited a robust sensing performance at a low working temperature, with rapid response and recovery. However, the sensitivity and limit of detection (LoD) of the sensor based on pure MoS2 nanosheets need to be improved.  In order to comprehensively evaluate the gas-sensing performance of MoS 2 nanosheets obtained by grinding with NMP, the performances of the MoS 2 nanosheet-based sensors were compared ( Table 1). As shown in Table 1, the response time and recovery time of MoS 2 nanosheets obtained by grinding with NMP for 50 ppm CH 2 O were 18 s and 0.51 s, respectively, which were close to the shortest response time (11 s) and recovery time (8 s) shown by ZnS and In 2 O 3 /MoS 2 [42,43]. Nevertheless, compared with the operating temperature (295 • C) of ZnS, the operating temperature of MoS 2 nanosheets was at room temperature (25 • C). Therefore, the MoS 2 nanosheets exhibited a robust sensing performance at a low working temperature, with rapid response and recovery. However, the sensitivity and limit of detection (LoD) of the sensor based on pure MoS 2 nanosheets need to be improved.  Figure 7 shows the I-V curves of S1 and S2 measured with an applied bias voltage ranging from −2 to 2 V at 1000 ppm CH 2 O. The I-V curves demonstrated a good ohmic contact between the sensing layers and the electrodes for both samples, which indicates that the sensor response was attributed to the sensitive material and not the metal-semiconductor contact.  Figure 7 shows the I-V curves of S1 and S2 measured with an applied bias voltage ranging from −2 to 2 V at 1000 ppm CH2O. The I-V curves demonstrated a good ohmic contact between the sensing layers and the electrodes for both samples, which indicates that the sensor response was attributed to the sensitive material and not the metal-semiconductor contact.
Nanomaterials 2022, 12, 4485 9 of 12 The sensing mechanism of MoS 2 nanosheet to CH 2 O, C 3 H 6 O, C 2 H 6 O, and 98%RH have been well-studied and described elsewhere [52][53][54][55]. According to these references, MoS 2 -nanosheets-based gas sensors exhibit n-type characteristics in our work. The possible sensing mechanism is as follows: The transfer of electrons from the conduction band to chemisorbed oxygen decreases the carrier density and increases the depletion layer, thereby increasing the resistance of the MoS 2 nanosheets. At room temperature, when the MoS 2 -nanosheet-based sensor is exposed to the target gas, for example, CH 2 O, the gas is adsorbed on the surface of the MoS 2 nanosheets. These chemisorbed molecules react with O − 2 (ads) to form H 2 O and CO 2 . Therefore, the trapped electrons are released back into the MoS 2 nanosheets, which increases the number of conductive channels, leading to a decrease in sensor resistance (Figure 8). The sensing mechanism of MoS2 nanosheet to CH2O, C3H6O, C2H6O, and 98%RH have been well-studied and described elsewhere [52][53][54][55]. According to these references, MoS2-nanosheets-based gas sensors exhibit n-type characteristics in our work. The possible sensing mechanism is as follows: The transfer of electrons from the conduction band to chemisorbed oxygen decreases the carrier density and increases the depletion layer, thereby increasing the resistance of the MoS2 nanosheets. At room temperature, when the MoS2-nanosheet-based sensor is exposed to the target gas, for example, CH2O, the gas is adsorbed on the surface of the MoS2 nanosheets. These chemisorbed molecules react with O 2 − (ads) to form H2O and CO2. Therefore, the trapped electrons are released back into the MoS2 nanosheets, which increases the number of conductive channels, leading to a decrease in sensor resistance (Figure 8).

Conclusions
MoS2 nanosheets were prepared with two grinding solvents via grinding-assisted liquid-phase exfoliation. The effects of grinding solvents on the structure of MoS2 nanosheets as well as the gas-sensing performance were studied. The structural and gassensing properties of MoS2 were investigated using XRD, SEM, and Raman spectroscopy. The sensing performance of MoS2 toward four target gases, including CH2O, C3H6O, C2H6O ,and 98% RH, was analyzed at room temperature. The experimental results proved that the MoS2 nanosheets exfoliated with NMP responded better than the MoS2 nanosheets exfoliated with ACN although NMP was not removed completely. The MoS2 nanosheet-based sensor also exhibited excellent response. However, the sensitivity and LoD of the sensor need to be improved. Accordingly, although NMP cannot be removed completely from the surface of MoS2, NMP exhibits good gas sensitivity compared with other materials. Therefore, NMP is preferred for the preparation of gas-sensitive materials in grinding-assisted liquid-phase exfoliation. The results provide an experimental basis for the preparation of two-dimensional materials and their application in gas sensors.
Author Contributions: H.W. designed the experiments, analyzed the data, and wrote the paper; X.X. performed the theoretical analysis; T.S. edited the manuscript and supervised the study. All authors have read and agreed to the published version of the manuscript.

Conclusions
MoS 2 nanosheets were prepared with two grinding solvents via grinding-assisted liquid-phase exfoliation. The effects of grinding solvents on the structure of MoS 2 nanosheets as well as the gas-sensing performance were studied. The structural and gas-sensing properties of MoS 2 were investigated using XRD, SEM, and Raman spectroscopy. The sensing performance of MoS 2 toward four target gases, including CH 2 O, C 3 H 6 O, C 2 H 6 O, and 98% RH, was analyzed at room temperature. The experimental results proved that the MoS 2 nanosheets exfoliated with NMP responded better than the MoS 2 nanosheets exfoliated with ACN although NMP was not removed completely. The MoS 2 nanosheet-based sensor also exhibited excellent response. However, the sensitivity and LoD of the sensor need to be improved. Accordingly, although NMP cannot be removed completely from the surface of MoS 2 , NMP exhibits good gas sensitivity compared with other materials. Therefore, NMP is preferred for the preparation of gas-sensitive materials in grinding-assisted liquid-phase exfoliation. The results provide an experimental basis for the preparation of two-dimensional materials and their application in gas sensors.
Author Contributions: H.W. designed the experiments, analyzed the data, and wrote the paper; X.X. performed the theoretical analysis; T.S. edited the manuscript and supervised the study. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by (National Natural Science Foundation of China) grant number (62061046, 51403180) and (The Third "Tianshan Talents" Training Project of Xinjiang Uygur Autonomous Region).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors have no conflict of interest to declare.